The invention relates to a video camera using a solid state image pickup device and, more particularly, to a function to correct defective pixels of a solid state image pickup device by signal processes.
As a conventional video camera having a function to correct defective pixels of a solid state image pickup device, for example, a video camera disclosed in JP-A-3-236689 can be mentioned. According to such a video camera, after an image signal obtained by the solid state image pickup device was converted into a digital signal, in order to correct an abnormal signal of an image, what is called a white spot noise, due to defective pixels, whether the level of the absolute value of an outline correction signal is equal to or larger than a predetermined value or not is discriminated by using an outline correction circuit, thereby automatically judging whether the pixels are defective pixels or not.
Although the solid state image pickup device converts an energy of light into an electric signal, it has a nature to also convert a thermal energy into an electric signal. The signal component which was converted into the electric signal by such a thermal energy is caused by a dark current. Since the dark current occurs by the thermal energy, a temperature dependency is strong and its value is almost doubled when the temperature rises by 10.degree. C.
A white spot noise occurs when the dark current is abnormally increased as compared with the other pixels due to defective pixels. Therefore, the signal level of the white spot noise has a temperature dependency. Since the video camera executes an automatic gain control (AGC), when an object is dark, a gain of circuit system is raised, thereby keeping a brightness of a video image constant. Thus, the level of white spot noise is also changed by the gain of the AGC. Further, since a change similarly occurs with regard to the normal signal component, it is very difficult to detect the white spot noise in the ordinary photographing state. In the above literature, however, a judgment reference of the white spot noise is merely set to a predetermined value, so that no consideration is made in the conventional video camera with respect to a point that the white spot noise due to defective pixels is detected at a high precision.
Reference may further be made to U.S. Pat. No. 4,654,714 which relates to correction of white spot noise.